Thread-safety: make all DTConfigManager::getDTConfig..() const to return a const pointer
thread-safety: return const* for getDTConfigBti and  getDTConfigTraco too
Merge pull request #1420 from smuzaffar/thread-safety-fix-L1TriggerConfig-DTTPGConfig

Multithreading fix -- L1TriggerConfig/DTTPGConfig: Const function returns pointer or reference to non-const object